Here is some information on impacted crops and what my cause it and how to cure.
The crop may occasionally become impacted when feed is withheld prior to worming, causing chickens to eat too much afterwards. Crops may also get packed when birds are free ranged where little is available to eat but tough, fibrous vegetation. Even if the bird continues to eat nutrition cannot get through. The swollen crop may cut off the windpipe, suffocating the bird. Crop impaction is not likely to occur in properly fed birds.
Provide proper rations and plenty of clean, fresh water. If feed is withheld prior to worming, offer a moistened ration 1-hour after worming.
If you suspect impacted crop, administer two eye-droppers full of mineral oil (NOT MINERAL SPIRITS!) by forcing an eye-dropper down the chickens gullet, and massage the hard lump between your fingers to soften it up and help her pass it. Repeat if necessary the following day. If the crop remains hard and the chicken appears distressed, then you may resort to the above treatment or take it to a vet.
Good luck,